Dr. Yuzo Kawaji was a scientist in Hiroshima who worked under Dr. James Bowen in his studies about radiation poisoning. When they discovered a boy who had absorbed massive amounts of radiation as a result of the Hiroshima bombings but had not died, they were intrigued and wondered if the child might have grown from the disembodied but still living heart of Dr. Victor Frankenstein's monster. A former Japanese naval officer named Kawai imparted this information to the doctors.
Kawaji undertook a trip to Germany to speak to Dr. Liesendorf, the former Nazi scientist who had originally studied the heart. Liesendorf told him that the only way to prove if the boy was Frankenstein was to amputate his limbs and see if they grew back. When Kawaji returned to Japan, both Bowen and their colleague Dr. Sueko Togami were opposed to this idea. However Kawaji attempted to do it in the middle of the night, but was interrupted by reporters, and the commotion they caused resulted in Frankenstein breaking free and escaping.
Later, during the scientists' race to find Frankenstein before the military did, Kawaji revealed his intention to destroy Frankenstein with bombs, despite the objections of Bowen and Togami. But instead of Frankenstein, Kawaji found himself facing Baragon, who attacked him. Kawaji fell down a cliff and was injured, but the kind-hearted Frankenstein fought off Baragon and retrieved the wounded scientist, returning him safely to Bowen and Togami. Kawaji thereafter regretted his earlier desire to slay Frankenstein.